Nerve Impulses
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Resting Membrane Potential Action Potentials How impulses start (receptors) Propagation of Impulses Speed of Impulses

 

Neurones send messages electrochemically; this means that chemicals (ions) cause an electrical impulse. Neurones and muscle cells are electrically excitable cells, which means that they can transmit electrical nerve impulses. These impulses are due to events in the cell membrane, so to understand the nerve impulse we need to revise some properties of cell membranes.

 

The Resting Membrane Potential  [back to top]

When a neurone is not sending a signal, it is at ‘rest’.  The membrane is responsible for the different events that occur in a neurone.  All animal cell membranes contain a protein pump called the sodium-potassium pump  (Na+K+ATPase). This uses the energy from ATP splitting to simultaneously pump 3 sodium ions out of the cell and 2 potassium ions in. 

The Sodium-Potassium Pump (Na+K+ATPase)
(Provided by:  Doc Kaiser's Microbiology Website)

Three sodium ions from inside the cell first bind to the transport protein. Then a phosphate group is transferred from ATP to the transport protein causing it to change shape and release the sodium ions outside the cell. Two potassium ions from outside the cell then bind to the transport protein and as the phospate is removed, the protein assumes its original shape and releases the potassium ions inside the cell.

If the pump was to continue unchecked there would be no sodium or   potassium ions left to pump, but there are also sodium and potassium ion channels in the membrane. These channels are normally closed, but even when closed, they “leak”, allowing sodium ions to leak in and potassium ions to leak out, down their respective concentration gradients.

Concentration of ions inside and outside the neurone at rest:

Ion Concentration inside cell/mmol dm-3 Concentration outside cell/mmol dm-3 Why don’t  the ions move down their concentration gradient?
K+ 150.0 2.5 K+ ions do not move out of the neurone down their concentration gradient due to a build up of positive charges outside the membrane.  This repels the movement of any more K+ ions out of the cell.
Na+ 15.0 145.0
Cl- 9.0 101.0 The chloride ions do not move into the cytoplasm as the negatively charged protein molecules that cannot cross the surface membrane repel them.

The combination of the Na+K+ATPase pump and the leak channels cause a stable imbalance of Na+ and K+ ions across the membrane.  This imbalance of ions causes a potential difference (or voltage) between the inside of the neurone and its surroundings, called the resting membrane potential. The membrane potential is always negative inside the cell, and varies in size from –20 to –200 mV (milivolt) in different cells and species (in humans it is –70mV). The Na+K+ATPase is thought to have evolved as an osmoregulator to keep the internal water potential high and so stop water entering animal cells and bursting them. Plant cells don’t need this as they have strong cells walls to prevent bursting.

 

Check Point g  The Resting Membrane Potential is always negative (-70mV)

  • K+ pass easily into the cell
  • Cl- and Na+ have a more difficult time crossing
  • Negatively charged protein molecules inside the neurone cannot pass the membrane
  • The Na+K+ATPase pump uses energy to move 3Na+ out for every 2K+ into neuron
  • The imbalance in voltage causes a potential difference across the cell membrane - called the resting potential

 

The Action Potential  [back to top]

The resting potential tells us about what happens when a neurone is at rest.  An action potential occurs when a neurone sends information down an axon.  This involves an explosion of electrical activity, where the nerve and muscle cells resting membrane potential changes.

In nerve and muscle cells the membranes are electrically excitable, which means they can change their membrane potential, and this is the basis of the nerve impulse. The sodium and potassium channels in these cells are voltage-gated, which means that they can open and close depending on the voltage across the membrane.

The normal membrane potential inside the axon of nerve cells is –70mV, and since this potential can change in nerve cells it is called the resting potential. When a stimulus is applied a brief reversal of the membrane potential, lasting about a millisecond, occurs. This brief reversal is called the action potential:

An action potential has 2 main phases called depolarisation and repolarisation:

At rest, the inside of the neuron is slightly negative due to a higher concentration of positively charged sodium ions outside the neuron. 
When stimulated past threshold (about –30mV in humans), sodium channels open and sodium rushes into the axon, causing a region of positive charge within the axon.  This is called depolarisation 
The region of positive charge causes nearby voltage gated sodium channels to close. Just after the sodium channels close, the potassium channels open wide, and potassium exits the axon, so the charge across the membrane is brought back to its resting potential.  This is called repolarisation
This process continues as a chain-reaction along the axon.  The influx of sodium depolarises the axon, and the outflow of potassium repolarises the axon. 
The sodium/potassium pump restores the resting concentrations of sodium and potassium ions 

 

 

 

(provided by: Markham)

 

 

 

Check Point  g  Action Potential has two main phases:

 Depolarisation. A stimulus can cause the membrane potential to change a little. The voltage-gated ion channels can detect this change, and when the potential reaches –30mV the sodium channels open for 0.5ms. The causes sodium ions to rush in, making the inside of the cell more positive. This phase is referred to as a depolarisation since the normal voltage polarity (negative inside) is reversed (becomes positive inside).
Repolarisation. At a certain point, the depolarisation of the membrane causes the sodium channels to close.  As a result the potassium channels open for 0.5ms, causing potassium ions to rush out, making the inside more negative again. Since this restores the original polarity, it is called repolarisation.  As the polarity becomes restored, there is a slight ‘overshoot’ in the movement of potassium ions (called hyperpolarisation).  The resting membrane potential is restored by the Na+K+ATPase pump.

 

 

‘All or Nothing’ Law

The action potential only occurs if the stimulus causes enough sodium ions enter the cell to change the membrane potential to a certain threshold level.  At the threshold, sodium gates open in the membrane and allow a sudden flood of sodium ions to enter the cell.   If the depolarisation is not great enough to reach the threshold, then an action potential (and hence an impulse) will not be produced.  This is called the all or nothing law.   This means that the ion channels are either open or closed; there is no half-way position.  This means that the action potential always reaches +40mV as it moves along an axon, and it is never attenuated (reduced) by long axons.   Action potentials are always the same size, however the frequency of the impulse carrying the information can determine the intensity of the stimulus, i.e. strong stimulus = high frequency.

 

How do Nerve Impulses Start?  [back to top]

We and other animals have several types of receptors of mechanical stimuli. Each initiates nerve impulses in sensory neurons when it is physically deformed by an outside force such as:

Mechanoreceptors enable us to

E.g. Touch

Light touch is detected by receptors in the skin. These are often found close to a hair follicle so even if the skin is not touched directly, movement of the hair is detected.

In the mouse, light movement of hair triggers a generator potential in mechanically-gated sodium channels in a neuron located next to the hair follicle. This potential opens voltage-gated sodium channels and if it reaches threshold, triggers an action potential in the neuron.

Touch receptors are not distributed evenly over the body. The fingertips and tongue may have as many as 100 per cm2; the back of the hand fewer than 10 per cm2.   This can be demonstrated with the two-point threshold test. With a pair of dividers like those used in mechanical drawing, determine (in a blindfolded subject) the minimum separation of the points that produces two separate touch sensations. The ability to discriminate the two points is far better on the fingertips than on, say, the small of the back.

The density of touch receptors is also reflected in the amount of somatosensory cortex in the brain assigned to that region of the body.

Proprioception

The Pacinian Corpuscle

  Pacinian corpuscles are pressure receptors. They are located in the skin and also in various internal organs. Each is connected to a sensory neuron.   Pacinian corpuscles are fast-conducting, bulb-shaped receptors located deep in the dermis.  They consist of the ending of a single neurone surrounded by lamellae. They are the largest of the skin's receptors and are believed to provide instant information about how and where we move. They are also sensitive to vibration. Pacinian corpuscles are also located in joints and tendons and in tissue that lines organs and blood vessels. 

 Pressure on the skin changed the shape of the Pacinian corpuscle.  This changes the shape of the pressure sensitive sodium channels in the membrane, making them open.  Sodium ions diffuse in through the channels leading to depolarisation called a generator potential.  The greater the pressure the more sodium channels open and the larger the generator potential.  If a threshold value is reached, an action potential occurs and nerve impulses travel along the sensory neurone.  The frequency of the impulse is related to the intensity of the stimulus.

Adaptation

When pressure is first applied to the corpuscle, it initiates a volley of impulses in its sensory neuron. However, with continuous pressure, the frequency of action potentials decreases quickly and soon stops. This is the phenomenon of adaptation.

Adaptation occurs in most sense receptors. It is useful because it prevents the nervous system from being bombarded with information about insignificant matters like the touch and pressure of our clothing.

Stimuli represent changes in the environment. If there is no change, the sense receptors soon adapt. But note that if we quickly remove the pressure from an adapted Pacinian corpuscle, a fresh volley of impulses will be generated.

The speed of adaptation varies among different kinds of receptors. Receptors involved in proprioception - such as spindle fibres - adapt slowly if at all.

Check Point g  The Pacinian Corpuscle

Deforming the corpuscle creates a generator potential in the sensory neuron arising within it. This is a graded response: the greater the deformation, the greater the generator potential. If the generator potential reaches threshold, a volley of action potentials (also called nerve impulses) are triggered at the first node of Ranvier of the sensory neuron.

Overall:
In living cells nerve impulses are started by receptor cells. These all contain special sodium channels that are not voltage-gated, but instead are gated by the appropriate stimulus (directly or indirectly). For example:  

  • Chemical-gated sodium channels in tongue taste receptor cells open when a certain chemical in food binds to them hemical-gated sodium channels in tongue taste receptor cells open when a certain chemical in food binds to them

  • Mechanically-gated ion channels in the hair cells of the inner ear open when they are distorted by sound vibrations

In each case the correct stimulus causes the sodium channel to open

i

Causes sodium ions to flow into the cell

i

Causes a depolarisation of the membrane potential

i

Affects the voltage-gated sodium channels nearby and starts an action potential (providing the threshold point is reached) 

 

 

Check Point g  An action occurs due to an increase in membrane permeability to Na+

  • An action potential is initiated by receptor cells that cause sodium channels to open
  • An action potential can only occur if the depolarisation of the membrane reaches the threshold point.  This is the all or nothing law.

How are Nerve Impulses Propagated?  [back to top]

Once an action potential has started it is moved (propagated) along an axon automatically. The local reversal of the membrane potential is detected by the surrounding voltage-gated ion channels, which open when the potential changes enough.

The ion channels have two other features that help the nerve impulse work effectively:

The refractory period is necessary as it allows the proteins of voltage sensitive ion channels to restore to their original polarity.

Check Point g  Nerve Impulses travel in one direction

  • This is due to the refractory period i.e. Na+ channels are open or recovering.

  •   The refractory period allows the voltage sensitive ion channels to restore their original polarity

    • Absolute refractory period - Na+ channels are open or recovering

    • Relative refractory period - Occurs when the membrane is hyperpolarised (-80mV), where the K+ channels are open.  Action potentials are more difficult to generate during this period relative to resting potential (-70mV)

 


How Fast are Nerve Impulses?  [back to top]

Action potentials can travel along axons at speeds of 0.1-100 m/s. This means that nerve impulses can get from one part of a body to another in a few milliseconds, which allows for fast responses to stimuli. (Impulses are much slower than electrical currents in wires, which travel at close to the speed of light, 3x108 m/s.) The speed is affected by 3 factors:

 

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Last updated 17/04/2004